Great attention is now paid to optical recording media capable of high density recording and erasing the once recorded information for rewriting. Among such rewritable optical recording media, phase change recording media are designed such that recording is performed by irradiating a laser beam to the recording layer to change its crystalline state and reading is performed by detecting the change of reflectivity of the recording layer associated with that state change. The phase change recording media are of greater interest because modulating the intensity of a single laser beam enables overwriting and the drive unit used for their operation may have a simple optical system as compared with that used for magneto-optical recording media.
For the phase change recording layer, chalcogenide materials such as Ge—Te and Ge—Sb—Te base materials are often used because of a greater difference in reflectivity between the crystalline and amorphous states and a relatively high stability in the amorphous state.
In phase change media with overwriting capability, amorphous recorded marks are formed by irradiating a laser beam at the recording power level to the crystalline recording layer for melting the irradiated spot, and rapidly cooling from the molten state. Erasion is carried out by irradiating a laser beam at the erasing power level to the recording layer to heat it to a temperature equal to or above its crystallization temperature, but below its melting point and then slowly cooling for allowing the amorphous recorded marks to crystallize. Then overwriting becomes possible by irradiating a single laser beam while modulating its intensity. The rate-determining factor in high speed recording of such phase change media is the crystallization speed of the recording layer, that is, the crystal transformation speed at which the material transforms from amorphous to crystalline. For high speed recording, a recording layer having a high crystallization speed may be used. At too high a crystallization speed, amorphous recorded marks become unstable due to the enhanced tendency to crystallize. Then, recording is often accompanied by cross-erasing that recorded marks in adjacent tracks are erased as well.
As a consequence, for the overwritable phase change media, it is prohibited to markedly increase the crystallization speed of the recording layer. This makes it difficult to significantly increase the data transfer rate. The cross-erasing can be reduced by increasing the arrangement pitch of recording tracks (referred to as track pitch), regrettably at the sacrifice of recording density.